In a communication process encrypted by using a quantum key distribution (QKD) process. Usually, on a transmit end, a group of true random number sequences are first generated by using a physical process, and the group of true random number sequences are encoded onto a group of quantum states, and sent to a receive end through a quantum channel. On the receive end, the quantum states are randomly measured according to a protocol that is specified in advance, and then the receive end and the transmit end perform screening and negotiation by using a measurement base used in typical channel comparison, to generate a secure key, and finally encrypt to-be-sent plaintext by using the key for communication.
QKD includes a technological branch being continuous-variable quantum key distribution (CV-QKD). An existing quantum transmit end in CV-QKD is shown in FIG. 1, and the quantum end is configured to generate quantum light, and send the quantum light to a quantum receive end. The quantum light includes signal light. On the quantum transmit end, a random number to be sent to the quantum receive end is modulated by means of amplitude modulation and phase modulation onto regular components of the signal light.
The quantum light sent by the quantum transmitter further includes reference light whose light intensity is higher than a light intensity of the signal light. The reference light is used to help the quantum receive end detect a frequency offset and a phase difference between the signal light sent by the quantum transmitter and local oscillator light generated by the quantum receive end, to restore, in a digital signal processing (DSP) process of the quantum receiver, the random number sent by the quantum transmit end. The signal light and the reference light are pulse light whose periods are the same and time sequences are alternate. On the quantum receive end, homodyne detection needs to be performed on the signal light and the reference light and the local oscillator light. Therefore, polarization states of the signal light and the quantum light are the same and need to be the same as a polarization state of the local oscillator light.
The structure of the quantum receiver is shown in FIG. 1. The quantum receiver 10 includes a local oscillator light generator 11, a dynamic polarization controller 12, a 2:2 coupler 13, a balanced receiver 14, and a DSP 15.
The local oscillator light generator 11 is configured to generate the local oscillator light. The dynamic polarization controller 12 is configured to receive the quantum light sent by the quantum transmitter. Because the polarization state of the quantum light is changed during transmission of the quantum light, the polarization state of the quantum light received by the quantum receiver is indefinite. The dynamic polarization controller 12 needs to adjust polarization state of the quantum light, so that polarization state of the reference light and the signal light in the quantum light is the same as polarization state of the local oscillator light generated by the quantum receiver.
The local oscillator light generated by the local oscillator light generator 11 and the signal light emergent from the dynamic polarization controller 12 are incident to the 2:2 coupler 13 together. The 2:2 coupler 13 is configured to: evenly split the local oscillator light into first local oscillator light and second local oscillator light, and evenly split the signal light into a first pulse sequence and a second pulse sequence; and combine the first local oscillator light and the first pulse sequence into a first beam to be emergent, and combine the second local oscillator light and the second pulse sequence into a second beam to be emergent. The first beam and the second beam enter the balanced receiver 14, and balanced homodyne detection is performed on the first beam and the second beam, to detect the regular components of the signal light by means of the homodyne detection, so as to restore the random number to be sent by the quantum transmitter to the quantum receiver.
However, in the quantum receiver shown in FIG. 1, the dynamic polarization controller cannot rapidly perform adjustment during actual application, affecting use efficiency of the quantum receiver, and costs of the dynamic polarization controller are relatively high.